The current trend in high power laser technology is away from gas-transport devices, convectively cooled by fast axial or transverse flow, to non-flow systems cooled by diffusion. The advantages derived from the switch from convention to diffusion cooling are major reductions in size, complexity and cost.
In this context, recent research and development work presented in the scientific and trade literature has documented that a new generation of simple, compact and relatively low cost CO2 laser systems can be realized through the adoption of extended-area, narrow-gap diffusion-cooled discharge structures, excited by RF energy sources.
Recent widened and lengthened versions of these single-slot RF driven devices, generally known generically as slab discharge lasers, have demonstrated sealed-off 10.6 micron output performance from a few watts up to the kW level and from physical packages an order of magnitude smaller than conventional gas transport machines. However, further scaling in output power from even wider single slabs has proven problematic; not only because of difficulties in impedance matching and uniform RF power deposition within these greatly extended area single-electrode gain media, but also because the elevated beam power density inherent in a single very-wide-slab becomes unmanageable for both mirrors and windows.
An additional troublesome and costly problem encountered in very-wide-slab devices is thermal distortion of the gain channel with concomitant mode instability, due to single-sided electrode heating and cooling. The progressively serious beam asymmetry and nonuniformity encountered in these much wider discharge structures also degrades the laser's effectiveness for precision industrial processing sequences, such as cutting and welding.
From the scenario above, it has become increasingly clear that the attainment of high quality laser output radiation at the multi-kilowatt level is not viable from a single very-wide-slab diffusion cooled device. Fortunately in this context, it has recently been demonstrated that the output power limitation of a single-slab laser device can be negated through adoption of an array excitation and optical extraction concept. The technique features a multi-gain channel approach, in which a large number of narrow-gap discharge geometries are incorporated into a single array optical system. When the individual beamlets from an array comprised of N laser channels are combined or stacked to form a single beam, a system having N times the average output power is achieved. In this manner an additional order of magnitude reduction in size or increase in power per unit volume can be realized. In the event that the individual lasers in the array can be uni-phase-locked, the focused intensity or brightness of the composite beam increases as N.sup.2.
RF EXCITED PHASE LOCKED ARRAYS:
Thus, a major challenge in narrow-gap diffusion cooled gas discharge pumped laser technology has become the design of multi-channel laser array geometries that are compatible with efficient RF power deposition, beamlet stacking, and uni-phase-locking. Although linear 1 and 2 dimensional arrays have recently been examined, beam stacking and phase locking problems render their outputs less than desirable.
RADIAL ARRAY:
However, a radial geometry version of this multi-channel array approach, designated as the Zodiac (radial array) Laser structure, and documented in my U.S. Pat. No. 5,029,173 appears highly desirable, in terms of ease of manufacture, RF power deposition and phase locking. The compact and fully symmetric geometry also inherently negates temperature induced mechanical instabilities in both RF excitation and optical energy extraction subsystems.
Small scale, proof-of-concept experiments have demonstrated superior performance in cutting applications. Excellent performance has also been achieved even without phase locking, because of a major enhancement in output beam symmetry and reduction in focal spot size, due to a beneficial RADIAL BEAM STACKING phenomenon.
The concept is believed scalable to enormously high average and pulsed powers, and possibly to many hundreds of kilowatts, in an unusually small, sealed-off package, having no moving parts.
RADIAL ARRAY RF EXCITATION:
An object of this patent disclosure is therefor to teach the efficient and cost effective extension of the basic multi-channel array concept into the realm of high powered laser devices through utilization of electromagnetic resonant cavity techniques for integrated generation, power division and impedance matching of RF energy into a multiplicity of electrodes comprising a laser array of arbitrary geometry, and particularly for the radial geometry typical of the radial array laser system.
RADIAL ARRAY OPTICAL ENERGY EXTRACTION:
The approach also specifies DIFFRACTION-COUPLED-STABLE-RESONATOR and HYBRID-RESONATOR approaches for optical energy extraction. These new stable resonator optical extraction techniques may also be applied to other slab-type RF excited lasers, either single-channel or with multi-channel gain arrays, supporting a waveguide or non-waveguide mode of propagation, and using a variety of lasing gas mixtures such as CO2, CO, and Excimer. These optical concepts are also applicable to liquid and solid state laser gain media, pumped by either flashlamps or LED arrays.
There is thus provided in accordance with one aspect of the invention a laser system formed of plural slab type gain channels whose source of laser excitation energy for exciting the laser active media is coupled to the gain channels using a cavity type resonant circuit matched to each of the gain channels.
In one embodiment, the gain channels are confined by pairs of electrodes defining narrow-gap gas discharge regions and each pair of electrodes forms a transmission line. In another embodiment, the plural gain channels form a radial array having a common central axis and the resonant circuit has an axis of symmetry coinciding with the common central axis. The resonant circuit may form a resonant cavity, formed between inner and outer cylindrical electrical conductors. Such an embodiment preferably includes means at the end of each discharge channel to couple electrical energy back into the cavity and provide an impedance matched termination of the transmission lines formed by the electrodes, whereby upon excitation of the electrodes a traveling wave is formed. Preferably, magnetic loops disposed in the resonant cavity and electrically connected to the pairs of electrodes are used to couple energy to the electrodes from the cavity.
In a still further embodiment, the resonant circuit is formed from plural electrically conducting strips each forming a stripline excitation source.
The source of laser excitation energy preferably comprises a pair of co-axial re-entrant capacitively loaded concentric resonant cavities; and a vacuum tube electrically connected to the interior of the pair of coaxial re-entrant capacitively loaded concentric resonant cavities, thereby forming an rf oscillator, either mounted at one end of the cavity type resonant circuit or on the side of the cavity type resonant circuit to form a T. If the source is mounted on the side of the cavity type circuit, it may be formed from a multiplicity of vacuum tubes mounted on the side of the cavity type resonant circuit. In a still further embodiment, the source of laser excitation energy comprises a toroidal vacuum tube disposed within the cavity type resonant circuit and extending circumferentially about the radial array.
In a still further embodiment the invention provides a laser system comprising:
plural pairs of parallel electrodes faces, each pair of parallel electrodes faces defining a narrow-gap discharge channel:
the pairs of parallel electrode faces being arranged about the extending radially from a first common central axis:
means attached to said electrode faces for diffusion cooling of said electrode faces, such electrodes preferably being made from metallic or dielectric material selected for efficient heat transfer and having water cooling passages directly inside said electrode faces:
means attached to the electrode faces for providing laser excitation energy to each pair of electrode faces, such that multiple laser gain media are created between the multiplicity of narrow-gap electrode channels:
such multiple gap laser excitation energy being in the form of RF pumping energy derived from a power division and impedance transforming & matching system; comprising a resonant co-axial cavity structure, having a second central axis co-incident with the first common central axis, and mounted outside of and concentrically about the radial discharge channels:
means for supply RF pumping energy into the co-axial cavity impedance & power transforming system, from either an external high power RF generator source, or from integrated single or multiple RF amplifiers or self-excited cavity oscillators, built integrally into said co-axial impedance & power transforming resonant cavity system:
optical energy extraction means, having a third central axis coinciding with the first common central axis, and being disposed at either end of the pairs of parallel electrode faces for generating a common resonator mode for all of the narrow-gap gain channels simultaneously.
said optical extraction system comprising either: a toric resonator, with or without means for multiple phase-locked-feedback-loops and/or external master oscillator injection, for coherent or non-coherent multiple output beamlet coupling near the central axis,
or: a conventional unstable resonator, featuring coherent over-the-edge diffractive output beamlet coupling, with coherence means between the individual output beamlets being derived through injection-phase-locking from either an external master oscillator or from the oscillator-core region established at the central region of the unstable resonator:
In a further embodiment of the invention the optical energy extraction means includes a phase-coherent annular-coupled concave-convex stable resonator, having a common stable cavity mode:
means for optical energy extraction from each narrow-gap gain channel featuring peripheral diffractive output coupling as individual phase-locked beamlets, one for each radial gain channel and further comprising a plurality of identical small output windows disposed radially about and co-linearly with the multiplicity of gain channels:
means for heating-sinking the plurality of radially mounted optical extraction windows:
means for compacting the multiplicity of beamlets into a single composite optical output beam, utilizing an axicon, or means for simultaneously focusing all beamlets to a common point utilizing a common focusing system such as a Wolter lense:
In a further embodiment of the invention the optical energy extraction means features:
means for asymmetric aspect-ratio-compensation for the multiplicity of beamlets, utilizing conical beam expanding and re-collimating reflective elements:
means for stacking, compacting and re-collimating the aspect-ratio-compensated beamlets into a uniformly illuminated composite beam of high azimuthal symmetry and good beam quality of low M.sub.2 value:
In a still further embodiment of the invention, the radial gain channels are mounted in an iris-coupled stable toric optical resonator featuring:
means for coherent phase-locked optical energy extraction as an annulus near the centerline of the structure.
In a still further embodiment of the invention, the radial gain channels are mounted in a MOPA optical extraction system featuring:
a centrally located Master Core Oscillator stable resonator, concentrically surrounded by a plurality of walk-off Power Amplifier gain sections, each power amplifier section providing an individual output beamlet phase locked with the central master oscillator, via simultaneously injection from the common centrally located master core oscillator.
In a lower power miniature embodiment of the invention, multi-channel laser excitation is supplied in the form of RF pumping energy derived from an impedance transforming and matching system; comprising multiple strip-line RF resonators all mutually coupled and having a second central axis co-incident with the first common central axis, and mounted outside of and concentrically about the radial discharge channels:
means for supply RF pumping energy into these coupled strip line resonators, from either an external RF generator source, or from integrated single or multiple RF amplifiers or self-excited oscillators, built integrally with said impedance transforming resonant strip line systems:
means for extracting optical energy from all the gain channels simultaneously, such means featuring a common optical resonator, either a stable, unstable or hydride optical resonator configuration, with either non-phase locked or fully phase locked multiple beamlet extraction, the latter derived via external master oscillator injection of self injection operation.
In a much higher output power embodiment of the invention plural radial multi-channel gain sections are mounted concentrically about a common central axis coincident with a third common optical resonator common axis, such that several hundred individual gain channels simultaneously contribute to the same optical output mode:
means attached to the electrode faces for providing diffusion cooling and laser excitation energy to each pair of electrode faces, such that efficient multiple laser gain media are created within the multiplicity of narrow-gap electrode channels:
such multiple gap laser excitation energy being in the form of RF pumping energy derived from a multi-magnetic loop impedance transforming and matching system; comprising a resonant co-axial cavity structure, having a second central axis co-incident with the first common central axis, and mounted outside of and concentrically about the radial discharge channels:
means for supply RF power to this co-axial RF cavity structure featuring several very high power RF power vacuum triodes or tetrodes mounted radially and integrally with the cavity structure such that the entire assembly has a distinct resonant RF frequency at which RF power is either generated or amplified within said RF cavity, and subsequently coupled to each electrode segment via a multiplicity of independent magnetic loops:
means for extracting optical energy from all of the gain channels simultaneously, such means featuring a common optical resonator of either a stable, unstable or hydride optical resonator configuration, having either non-phase locked or fully phase locked multiple beamlet extraction, the latter derived via external master oscillator injection of self injection operation.